Conventional designs for feeding catalysts to the melt phase process for making polyester polymers call for using a catalyst mix tank and one feed tank system to the desired point on the melt phase process. The point of addition to the melt phase process depends upon the type of esterification, whether direct or ester exchange, and the type of catalyst employed, along with the desired characteristics of the polymer and the desired residence time. We have recently discovered that certain catalyst systems of alkaline earth metal or alkali (“M”) and aluminum can be retained in solution at ambient conditions using certain solvents, such as mono-ol ethers or polyhydroxylether solvents as further described below. These catalyst systems are difficult to dissolve in ethylene glycol and readily precipitate on standing at ambient conditions, especially when the molar ratio of M:Al approaches 1:1 and/or when the amount of Al in the solution exceeds 3000 ppm.
We have discovered that catalyst solutions which contain molar ratios of M:Al approaching 1:1 are desirable in order to minimize the formation of yellow color in the polyester polymer, and to provide a catalyst solution that is less hazy and more clear. A catalyst mix tank and single feed system to the melt phase process works well to produce polyester polymers containing the same ratio of M:Al, whether 1:1 or otherwise. However, it is may the case that one desires to produce a variety of polymers on a single manufacturing line. Depending on the nature of the change desired in the polymer, the change may call for adjusting the molar ratio of M:Al from one type of polyester polymer to another type of polyester polymer. In this case, it would be impractical to design a process which supplies only one fixed catalyst ratio to the melt phase process. Changing out the catalyst ratio in a single mix tank/feed system is impractical because an inventory of the previously used catalyst remains in the mix tank, and renders the process rather inflexible.
Multiple manufacturing lines are often present in a plant. One line may be set to produce one polyester polymer with a certain set of characteristics, while another line in simultaneous operation may be set to produce a polyester polymer with a different set of characteristics, each line requiring a different catalyst ratio of M:Al. Providing a different catalyst mixing tank for each line increases the capital cost of a plant.
It would be desirable to provide a catalyst feed system which does not require depleting the inventory in a catalyst mix tank, which allows rapid adjustments to the catalyst ratio so as to provide quicker change overs to produce different polyester polymers on the same line, or to make necessary catalyst adjustments in the event that a polymer for one reason or another starts to fall off specification on the same line, and/or to provide a less costly system to provide for one mix tank which will feed multiple lines at different catalyst ratios depending on the requirements of each line.